There are known pyrometallurgical methods for recovering lead values from lead sulphides, however, such treatment may entail expensive mining and beneficiation process steps to concentrate the sulphides and in addition it may create environmental problems, including the generation of sulphurous gases, lead bearing fumes, acid drainage and substantial land disturbance. On the other hand, hydrometallurgical treatment of lead sulphides with conventional mineral acids for lead recovery, may require several expensive process steps to overcome the low aqueous solubility of the resulting lead chloride or lead sulphate.
In-situ leaching has several economic advantages over conventional leaching which usually has to be performed in containers having costly corrosion resistant lining, or in expensive pressure vessels and similar equipment. The initial cost of in-situ leaching is relatively low, requiring only the rendering of the ore body accessible to a leach solution by conventional methods, such as providing passages to the ore body and/or allowing the leach solution to penetrate the bedrock by naturally occurring fissures and pores. The prepared passages, naturally occuring fissures and pores act as a manifold to distribute the leachant through the ore body and bring the leachant in contact with the value mineral, in the present instance, with the sulphides. The leachant is subsequently either circulated through the ore body by means of pumping or left in stationary contact for an appropriate period of time to solubilize the value metal. The resulting pregnant solution is then withdrawn for treatment in further recovery process steps. There are, however, very important aspects that need to be considered when a value metal recovery process by in-situ leaching is devised:
i) the formation of water-insoluble compounds of the metal to be recovered cannot be tolerated, since such insoluble compounds would form a coating on the mineral within the value metal ore deposit in the bedrock, thus rendering the ore inaccessible to further solubilization from the ore body; PA0 ii) the formation of an insoluble by-product of the leach reagent and the gangue minerals or other constituents of the bedrock, is to be avoided, since such insoluble by-product can easily lead to blocking of pores and fissures, thus effectively terminating the in-situ leaching process; PA0 iii) yet another aspect that requires serious consideration regarding the practice of in-situ leaching, is that the reagent used in the solubilization of the value metal be harmless to the environment, and that termination of the in-situ leaching process, be that temporary or otherwise, can be accomplished without creating any toxic compound.
U.S. Pat. No. 4,381,873 issued to Johnson et al. on May 3, 1983, describes a solution mining process for copper-iron sulphidic ores wherein the ore body is conventionally fractured, then roasted in-situ and subsequently leached with a sulphuric acid solution provided by means of water addition to the roasted ore in-situ and/or supplemented with a mineral acid. U.S. Pat. No. 4,381,873, however, is a process which could not be readily adapted to in-situ leaching of lead sulphidic minerals, as it would lead to insoluble lead sulphate coating formation in-situ, thus impeding the progress of the leaching reaction; moreover, the process would also result in insoluble calcium sulphate formation as a by-product, thus blocking naturally occurring pores and fissures, or the pores created by roasting. In addition, U.S. Pat. No. 4,381,873 would generate sulphur dioxide and harmful acidic ground water, both being considered environmentally highly undesirable.
There are conventional processes for solubilizing, purifying and refining lead in oxidic lead compounds which utilize solutions containing acetate ions, however, the problems inherent in the refining of oxidic lead compounds are quite different from those of recovering lead from mineral substances containing lead sulphides.
The conventional hydrometallurgical method for recovering value metals from their sulphides is to leach the sulphidic concentrate with a mineral acid, such as sulphuric or hydrochloric acid, or with an acidic solution of ferric chloride or sulphate, or with an acidic solution of copper sulphate or chloride, frequently assisted by an oxidizing agent. The leaching step is commonly conducted in a corrosion resistant container or an autoclave. The role of the oxidizing agent is to oxidize the sulphide ions in the sulphidic concentrate to sulphite or sulphate, thereby enhancing the leaching power of the leach liquor. Such a process is described, for example, in U.S. Pat. No. 3,959,436 issued to J. C. Watts on May 25, 1976. Most conventional leach processes utilizing mineral acids or solutions of sulphates or chlorides cannot be adapted to lead sulphide solubilization in a commercialy rewarding manner due to the low aqueous solubility of the resulting lead chloride and lead sulphate.
U.S. Pat. No. 3,933,973 issued to Evans et al. on Jan. 20, 1976, describes the treatment of finely ground lead sulphide containing mineral concentrates with an acetic acid-ammonium acetate solution in an autoclave at temperatures above 60.degree. C., in the presence of oxygen gas at 20-60 psi pressure. It is noted, that Evans et al. teach that below the above stated temperatures and pressures the oxidation reaction does not occur at practical rates. The object of the process of Evans et al. is to recover lead from a slurry of lead sulphide bearing concentrate of particle size smaller than 325 mesh and pulp density of less than 50 wt. %, by leaching the slurry in an expensive pressure resistant vessel. The source of the lead sulphide bearing concentrate is usually a lead sulphide containing ore which is first mined, then crushed, ground and subjected to costly mineral separation process steps to substantially remove the gangue minerals and other metal sulphidic minerals. In other words, the lead ore which is to be treated in the Evans et al. process needs to be of fairly high grade to render the process commercially rewarding.
It is further noted, that although the stated objective of Evans et al. is to stop the oxidation of sulphide at the elemental sulphur state, a considerable portion of the sulphur is oxidized to sulphate, thereby producing insoluble lead sulphate which is precipitated as fine particles in the course of the leaching process step with acetic acid-ammonium acetate solution. The lead sulphate by-product is removed from the slurry by subsequent process steps, such as treatment of the separated lead sulphate containing residue with ammonia in an autoclave. It is also to be noted that the process of U.S. Pat. No. 3,933,973 utilizes ammonium acetate in the pressure leaching step, and ammonia and similar ammonia based reagents in subsequent process steps; that is, ammonium ions appear to be the linchpin of the process steps of Evans et al. The use of no other metal acetate solution is suggested by Evans et al. Any gypsum, that is calcium sulphate, which may be precipitated as a consequence of oxidation of sulphides to sulphate can be eliminated in the liquid-solid separation step of the Evans et al. process.
Another aspect to be considered is that conventional recovery of lead from its ores, be it by a known pyrometallurgical processes or by a hydrometallurgical process such as described in U.S. Pat. No. 3,933,973, usually necessitates that the ore be brought to the surface by a mining operation often conducted at substantial depth below ground level, requiring costly equipment. The mined ore has to be crushed, ground, then usually subjected to beneficiation involving several mineral separation process steps, before it may be treated by conventional extractive processes. Observance of environmental regulations governing mining operations may further increase the cost of recovering lead from its ores by conventional processes.
There is a need for a hydrometallurgical method to recover lead values by in-situ solution mining of consolidated bedrock, or leaching lean sulphidic ores and tailings, more particularly grade lead sulphide ores, in unconsolidated minerals by the use of relatively inexpensive and environmentally acceptable reagents, without the application of expensive surface or underground mining process steps and costly leaching equipment.
Moreover, it is of importance that the lead be recovered from lead sulphidic ores in the absence of steps creating adverse environmental impact.